Arrangements of this kind are used, for example, for recording the speed of rotation or the speed in anti-lock systems. The sensor of such an arrangement, which may be constructed as a magnetic bridge arrangement, should be as free as possible from a sensor signal voltage in the non-detuned condition, i.e. in the stationary condition in the absence of a moved element, in order to ensure a high sensitivity of the arrangement. The stationary magnetic field of the pole surface of a magnet recorded by the sensor must therefore be symmetrical with respect to the sensor, which is substantially unattainable because of manufacturing tolerances and therefore leads to undesired offset voltages.
U.S. Pat. No. 4,745,363 discloses an arrangement, in which, by means of a Hall cell and a magnet, the teeth of a rotating toothed wheel are detected. A groove is provided in one of the pole faces of the magnet transverse to the direction of movement of the teeth of the toothed wheel in order to accommodate at least in part a Hall cell composed of a narrow semiconductor strip because the most sensitive plane detuned magnetically by the teeth of the toothed wheel lies at the area of the surface of the pole face. However, in spite of the groove, the Hall cell projects far beyond this area from the pole face, which is the reason why additional elements are provided for concentrating the magnetic field. The sensitivity of this arrangement strongly depends upon the accurate positioning of the Hall cell in the axis of symmetry of the magnet and is further influenced unfavourably by the projecting Hall cell and the comparatively great distance of the pole face from the toothed wheel connected therewith.
German Offenlegunschrift 34 26 784 A1 discloses an arrangement comprising a magnetoresistive sensor for indicating electrical signals. This sensor has a magnet comprising a central groove, which is aligned in the direction of movement of a toothed wheel and to which a magnetic measuring bridge arrangement is assigned. The groove serves to form an edge at the magnet, as a result of which essentially a homogeneous magnetic field component is to be produced in the plane of the pole face at right angles to the direction of movement. A magnetic detuning of this magnetic field component can then be detected by means of the measuring bridge arrangement. The measuring bridge arrangement is arranged for this purpose in such a manner centrally and at a certain distance from a pole face that it covers in part an edge of the groove. Since also in this arrangement positioning free from offset voltage is extremely difficult, two adjacent bridge arrangements are provided so that occurring offset voltages are to cancel each other. This complicated solution can also solve the problem of the positioning free from offset voltage of the measuring bridge arrangement only conditionally because for this purpose the offset voltages must have the same value, which is not always the case.
A further possibility to position a measuring bridge arrangement is the method of accurately surveying the magnetic field symmetry and then symmetrically mounting the measuring bridge arrangement in the surveyed area, which is also very complicated, however.
A further complicated method is the remagnetization of the magnet in order to obtain field symmetrization at the area of the sensor. For example, this requires a manipulation, which can vary the position of the sensor in the field of the pulse magnetization device then required.